1. FIELD OF THE INVENTION
This invention relates to wire enamels and components thereof, and more particularly to an imide-ring containing polyester, and wire enamels containing at least one polyester containing imide rings in the molecule.
2. DESCRIPTION OF THE PRIOR ART
The so-called polyester imide resins prepared by the introduction of imide rings into the polyester molecule have various excellent properties such as thermal life properties, heat shock resistance, solvent resistance, chemical resistance and super-charge resistance, and these resins recently have become extremely useful for wire enamels. In general, polyester imide resins are prepared as follows. A so-called imide ring containing carboxylic acid having terminal carboxylic groups such as ditrimellitic-imide acid is prepared by reaction of a tricarboxylic acid anhydride (a typical example of which is trimellitic anhydride) or a functional derivative thereof with a diamine. The resulting carboxylic acid is used as an acid component and is reacted with a polyhydric alcohol to form a polymer containing ester bonds. In this connection, polycarboxylic acids themselves containing imide rings in the molecule have extremely high melting points, and are insoluble in common solvents such as phenol, cresol and xylenol, and therefore, the synthesis of these polycarboxylic acids involves various disadvantages. For example, ditrimellitic-imide acid obtained by reaction of trimellitic anhydride wth diaminodiphenylmethane has a melting point of 360.degree. C or more, and therefore, when this imide acid is used as a part or all of the acid component and subjected to an esterification reaction, the reaction system is not uniform in the initial stages of the reaction. This results in the necessity for a relatively longer period of time as compared with a conventional esterification reaction. This becomes even more evident with increased degree of imidemodification. In addition, when starting materials having such high melting points are used, the reaction must be carried out very often in the presence of solvents, so as to improve the ability to stir the system during the reaction. However, in a reaction carried out at a relatively high temperature such as an esterification reaction, solution polymerization is preferably avoided if possible, condidering the boiling point of the solvent and the possiblity of interaction between the solvent and the monomer or polymer at high temperatures.
On the other hand, imide-modification improves the heat resistance of polyester resins with more improvement in the heat resistance being obtained as the degree of imidemodification increases, but, at the same time, an increase in the melting point of the polymers results as the degree of imidemodification increases. Accordingly, the modification degree is naturally limited from the standpoint of the ease of the production of the polymers and of the optimum viscosity thereof as wire enamels. From the stanpoint of conservation of resources the amount of solvent used in wire enamels should be as low as possible and to accomplish this the active ingredient in the wire enamel should be increased as much as possible and yet the viscosity of the wire enamel should be controlled to a viscosity which is suitable for common operations. In fact, from the viewpoint of electrical insulation, the active indgedient in wire enamel is not the solvent but resin component. In addition, the use of as small an amount of solvent as possible is advantageous in preventing environmental pollution. Thus, it is evident that the ideal situation would be operation in the absence of a solvent.